As is known, switched-mode voltage converters, which are preferred for their high efficiency and their reduced size as compared to classic linear converters, usually implement a self-supply technique that enables, starting from non-regulated input voltages, regulated output voltages to be obtained having an amplitude greater or smaller than the input voltage.
One of the most common types of switched-mode voltage converters is the isolated accumulation (“flyback”) type. A flyback voltage converter enables conversion of a first voltage value (present on an input of the converter) into a second voltage value (supplied on the output of the converter), maintaining the input and output of the converter galvanically isolated by the use of a transformer.
FIG. 1 shows a circuit diagram of a known voltage converter 1, of a flyback type.
The voltage converter 1 has an input 2 to which an input voltage Vin (for example, supplied by a rectifier circuit, not illustrated, starting from the mains voltage) is applied, and an output 3 supplying an output voltage Vout, and comprises a transformer 4, having a primary side and a secondary side, which is electrically isolated from the primary side. In particular, the transformer 4 has a primary winding 4a coupled to the input 2, a secondary winding 4b coupled to the output 3 by interposition of a first diode 6, and an auxiliary winding 4c (the latter set on the primary side of the transformer 4). An output capacitor 7 is coupled to the output 3. A main transistor 10, in particular an N-channel MOS transistor, is coupled between an internal node 8, which is in turn coupled to the primary winding 4a, and a reference terminal 9 (for example, a ground terminal). A bulk capacitor 11 is coupled between the input 2 and the reference terminal 9.
The voltage converter 1 further comprises: a PWM controller 12, used for regulation of the output voltage Vout, having a supply terminal 13, which receives a supply voltage Vcc and is coupled to the auxiliary winding 4c via the interposition of a second diode 14, and an output terminal, which is coupled to the gate terminal of the main transistor 10 and supplies a PWM signal for controlling opening and closing of the main transistor 10; and a self-supply circuit 15, having an input terminal coupled to the input 2 of the voltage converter 1, and an output terminal, which coincides with the supply terminal 13 of the PWM controller 12 and supplies the supply voltage Vcc.
In detail, the self-supply circuit 15 comprises: an accumulation capacitor 16, coupled between the supply terminal 13 and the reference terminal 9; and a start-up resistor 18 coupled between the input terminal 2 of the voltage converter and the supply terminal 13.
In a known way, the function of the self-supply circuit 15 is that of supplying the PWM controller 12 to enable it to regulate the output voltage Vout. In use, the accumulation capacitor 16 is initially charged by the input voltage Vin, through the start-up resistor 18. The PWM controller 12 switches on when the value of the voltage on the accumulation capacitor 16 reaches a first threshold value Vccon, for example, equal to 13.5 V. Next, the PWM controller 12 receives the supply voltage Vcc directly from the auxiliary winding 4c of the transformer 4.
The start-up resistor 18 is used in the initial turn-on phase (start-up) of the voltage converter 1 for supplying the turn-on supply to the PWM controller 12. However, a current flows through the start-up resistor 18 also at the end of the initial start-up phase, causing a considerable dissipation of power and reducing the efficiency of the voltage converter 1.
In addition, if the converter is used for regulating also an output current Iout, for example as a battery-charger, the auxiliary winding 4c is also used (in a known way that is not described in detail herein) for supplying a feedback signal to the PWM controller 12, for regulating both the output voltage Vout and the output current Iout. In this case, the voltage on the auxiliary winding 4c might not have a value sufficient for supplying the PWM controller 12. Consequently, also during the switching phase in which the PWM controller 12 is active, the PWM controller 12 is self-supplied through the start-up resistor 18, thus increasing the total power dissipation.
FIG. 2 shows a different circuit embodiment of the self-supply circuit 15 of the voltage converter 1 (the remaining elements of the voltage converter, which are present also in this embodiment, are not illustrated again here for clarity reasons).
In detail, the self-supply circuit 15 comprises: the accumulation capacitor 16 (previously described); an auxiliary transistor 21, in particular an N-channel MOS transistor having a drain terminal coupled to the input 2 of the voltage converter 1 and receiving the input voltage Vin; a first biasing resistor 22, having, for example, a value of resistance of 15 MΩ and coupled between the input 2 of the voltage converter 1 and the gate terminal of the auxiliary transistor 21; a second biasing resistor 23, coupled between the gate terminal of the auxiliary transistor 21 and the reference terminal 9; a current generator 24, which is coupled between the source terminal of the auxiliary transistor 21 and the supply terminal 13 of the PWM controller 12, via the interposition of a third diode 25, and has a control terminal; and a switch 26, coupled between the gate terminal of the auxiliary transistor 21 and the reference terminal 9.
The self-supply circuit 15 further comprises a control logic 28, having a first input coupled to the gate terminal of the auxiliary transistor 21, a second input coupled to the supply terminal 13, a first output supplying a control signal Vcc—OK to a control terminal of the switch 26, and a second output supplying to the control terminal of the current generator 24 an activation signal HV_EN.
In use, during a start-up phase, when the input voltage Vin (following upon progressive charging of the bulk capacitor 11, shown in FIG. 1) reaches a given threshold value, for example, equal to 80 V, the control logic 28 turns on the current generator 24 via the activation signal HV_EN, enabling a current Icharge to flow through the auxiliary transistor 21. This current Icharge, for example, having a value of 1 mA, charges the accumulation capacitor 16, raising the supply voltage Vcc across its terminals in a substantially linear way. When the supply voltage Vcc reaches the first threshold value Vccon, the signal Vcc—OK generated by the control logic 28 closes the switch 26, causing turning-off of the auxiliary transistor 21 and interruption of the flow of current Icharge through the same auxiliary transistor 21 and the current generator 24. The PWM controller 12 (FIG. 1) is then supplied by the energy stored in the accumulation capacitor 16, as long as the auxiliary winding 4c generates a voltage sufficiently high to sustain the operations of regulation of the controller.
The residual consumption of the self-supply circuit 15 is hence due only to the presence of the first biasing resistor 22, and is typically from 50 to 70 times lower than that of the circuit of FIG. 1.
The self-supply circuit 15 also intervenes for charging the accumulation capacitor 16 during the switching phase of the main transistor 10 (FIG. 1), in the case where the voltage on the auxiliary winding is not sufficient to supply the supply voltage Vcc, for example, in the case of operation as a battery-charger, when the battery is run down or in the presence of overload at the output. In detail, as soon as the supply voltage Vcc drops below a second threshold value Vccrestart, for example, equal to 10.5 V, the control logic 28 controls opening of the switch 26 by means of the signal Vcc—OK, and enables the current generator 24 by means of the signal HV_EN so as to charge the accumulation capacitor 16 via the current Icharge.
In order to contain costs, it is possible to integrate in one and the same chip (not illustrated) the auxiliary transistor 21 and the main transistor 10. In this case, as shown in FIG. 3, the auxiliary transistor 21 and the main transistor 10 share the drain terminal. The drain terminal is coupled to the internal node 8 (FIG. 1), which is in turn coupled to the primary winding 4a of the transformer 4, and is at a voltage which is not constant (i.e., which switches between a value of approximately 0 V and the value of the input voltage Vin).
The self-supply circuit 15 of FIG. 3 thus enables charging of the accumulation capacitor 16 only when the main transistor 10 is turned off, i.e., when the voltage of the aforesaid drain terminal (or, in a similar way of the internal node 8) is high and equal to the value of the input voltage Vin. Consequently, in the case where the self-supply circuit 15 is also used for self-supply of the PWM controller 12 through the accumulation capacitor 16 during the switching phase of the PWM controller 12, the current Icharge can charge the accumulation capacitor 16 only during the OFF phase of the switching period, when the voltage of the drain terminal is high. This condition can jeopardize proper operation of the self-supply circuit 15, especially for high values of duty cycle (higher than 50%) of the switching signal that regulates operation of the voltage converter 1, and consequently considerably limits the maximum value of duty cycle that can be obtained.
In fact, the auxiliary transistor 21 should be able to turn on rapidly during turning-off of the main transistor 10 in order to maximize the useful time (substantially corresponding to the OFF phase of the switching signal) for charging the accumulation capacitor 16. However, the switching rate of the auxiliary transistor 21 is limited by the gate capacitance of the latter and by the presence of the first biasing resistor 22, the value of which is commonly chosen high (for example, equal to 15 MΩ) in order to minimize the losses.
In use, when the main transistor 10 is turned on, the voltage on the internal node 8 is approximately 0 V and the auxiliary transistor 21 is off. When the main transistor 10 is turned off, the signal Vcc—OK generated by the control logic 28 controls opening of the switch 26, the voltage on the drain terminal of the auxiliary transistor 21 starts to increase, and the gate capacitor of the same auxiliary transistor 21 is charged, first by the injection of charge coming from the capacitance between the drain and gate terminals and then, when the voltage on the drain terminal reaches a sufficiently high value, through the biasing resistor 22. Both of these contributions of charge may not be, however, sufficient to turn on the auxiliary transistor 21 completely, and to supply the current Icharge required by the current generator 24, in a reasonable time. Consequently, a substantial part of the time available for charging the accumulation capacitor 16 may not be exploited. Therefore, in order to guarantee in any case the self-supply operation, it is hence common to limit the duty cycle to a value lower than 50%, for example equal to 45%.